How do I dump disk sectors?

I want to see the content of the sector 0 (mbr) of my hard drive. It is possible?

Yes, sure.

$ dd if=/dev/sda of=mbr bs=512 count=1
1+0 records in
1+0 records out
512 bytes (512 B) copied, 0.000188571 s, 2.7 MB/s

$ stat -c %s mbr
512

$ hd mbr
00000000  eb 48 90 d0 bc 00 7c 8e  c0 8e d8 be 00 7c bf 00  |.H....|......|..|
00000010  06 b9 00 02 fc f3 a4 50  68 1c 06 cb fb b9 04 00  |.......Ph.......|
00000020  bd be 07 80 7e 00 00 7c  0b 0f 85 10 01 83 c5 10  |....~..|........|
00000030  e2 f1 cd 18 88 56 00 55  c6 46 11 05 c6 46 03 02  |.....V.U.F...F..|
00000040  ff 00 00 20 01 00 00 00  00 02 fa 90 90 f6 c2 80  |... ............|
00000050  75 02 b2 80 ea 59 7c 00  00 31 c0 8e d8 8e d0 bc  |u....Y|..1......|
00000060  00 20 fb a0 40 7c 3c ff  74 02 88 c2 52 be 7f 7d  |. ..@|<.t...R..}|
00000070  e8 34 01 f6 c2 80 74 54  b4 41 bb aa 55 cd 13 5a  |.4....tT.A..U..Z|
00000080  52 72 49 81 fb 55 aa 75  43 a0 41 7c 84 c0 75 05  |RrI..U.uC.A|..u.|
00000090  83 e1 01 74 37 66 8b 4c  10 be 05 7c c6 44 ff 01  |...t7f.L...|.D..|
000000a0  66 8b 1e 44 7c c7 04 10  00 c7 44 02 01 00 66 89  |f..D|.....D...f.|
000000b0  5c 08 c7 44 06 00 70 66  31 c0 89 44 04 66 89 44  |\..D..pf1..D.f.D|
000000c0  0c b4 42 cd 13 72 05 bb  00 70 eb 7d b4 08 cd 13  |..B..r...p.}....|
000000d0  73 0a f6 c2 80 0f 84 ea  00 e9 8d 00 be 05 7c c6  |s.............|.|
000000e0  44 ff 00 66 31 c0 88 f0  40 66 89 44 04 31 d2 88  |[email protected]..|
000000f0  ca c1 e2 02 88 e8 88 f4  40 89 44 08 31 c0 88 d0  |[email protected]...|
00000100  c0 e8 02 66 89 04 66 a1  44 7c 66 31 d2 66 f7 34  |...f..f.D|f1.f.4|
00000110  88 54 0a 66 31 d2 66 f7  74 04 88 54 0b 89 44 0c  |.T.f1.f.t..T..D.|
00000120  3b 44 08 7d 3c 8a 54 0d  c0 e2 06 8a 4c 0a fe c1  |;D.}<.T.....L...|
00000130  08 d1 8a 6c 0c 5a 8a 74  0b bb 00 70 8e c3 31 db  |...l.Z.t...p..1.|
00000140  b8 01 02 cd 13 72 2a 8c  c3 8e 06 48 7c 60 1e b9  |.....r*....H|`..|
00000150  00 01 8e db 31 f6 31 ff  fc f3 a5 1f 61 ff 26 42  |....1.1.....a.&B|
00000160  7c be 85 7d e8 40 00 eb  0e be 8a 7d e8 38 00 eb  ||..}.@.....}.8..|
00000170  06 be 94 7d e8 30 00 be  99 7d e8 2a 00 eb fe 47  |...}.0...}.*...G|
00000180  52 55 42 20 00 47 65 6f  6d 00 48 61 72 64 20 44  |RUB .Geom.Hard D|
00000190  69 73 6b 00 52 65 61 64  00 20 45 72 72 6f 72 00  |isk.Read. Error.|
000001a0  bb 01 00 b4 0e cd 10 ac  3c 00 75 f4 c3 00 00 00  |........<.u.....|
000001b0  00 00 00 00 00 00 00 00  29 b6 ef 39 00 00 00 00  |........)..9....|
000001c0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
000001d0  01 6f 83 fe bf f6 af 35  1b 00 88 d8 9e 00 00 fe  |.o.....5........|
000001e0  ff ff 0f fe ff ff 2a 32  a2 01 97 58 ff 10 00 00  |......*2...X....|
000001f0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 55 aa  |..............U.|
00000200

See man dd and man hd for how this works.

And yeah. Be careful with dd! Don't abuse sudo; regular user's account with group disk membership is sufficient for reading blocks. If it says "permission denied" — then you may be trying to overwrite hard drive, take a closer look at what you are doing. If your MBR will get erased, you will lose your partition table. It will be hard to restore (albeit not impossible). Watch carefully your hard drive device: /dev/sda, don't confuse with /dev/sda1 or /dev/sdb. Be careful and don't rush. Read man dd. Good luck.

In addition to the proposed answer I would also suggest to use simply hd:

sudo hd /dev/sda -n 512 -s 0x0

00000000  eb 48 90 d7 bc 00 7a bb  a0 07 8e db 8e c3 be 00  |.H....z.........|
00000010  02 8b ce fc f3 a4 ea a3  00 a0 07 b9 04 00 8b fd  |................|
00000020  80 3d 80 74 05 83 c7 10  e2 f6 c3 b9 04 00 8b f5  |.=.t............|
00000030  88 2c 83 c6 10 e2 f9 c6  05 80 c3 60 b4 43 03 02  |.,.........`.C..|
00000040  ff 00 00 20 01 00 00 00  00 02 fa 90 90 f6 c2 80  |... ............|
00000050  75 02 b2 80 ea 59 7c 00  00 31 c0 8e d8 8e d0 bc  |u....Y|..1......|
00000060  00 20 fb a0 40 7c 3c ff  74 02 88 c2 52 be 7f 7d  |. ..@|<.t...R..}|
00000070  e8 34 01 f6 c2 80 74 54  b4 41 bb aa 55 cd 13 5a  |.4....tT.A..U..Z|
00000080  52 72 49 81 fb 55 aa 75  43 a0 41 7c 84 c0 75 05  |RrI..U.uC.A|..u.|
00000090  83 e1 01 74 37 66 8b 4c  10 be 05 7c c6 44 ff 01  |...t7f.L...|.D..|
000000a0  66 8b 1e 44 7c c7 04 10  00 c7 44 02 01 00 66 89  |f..D|.....D...f.|
000000b0  5c 08 c7 44 06 00 70 66  31 c0 89 44 04 66 89 44  |\..D..pf1..D.f.D|
000000c0  0c b4 42 cd 13 72 05 bb  00 70 eb 7d b4 08 cd 13  |..B..r...p.}....|
000000d0  73 0a f6 c2 80 0f 84 ea  00 e9 8d 00 be 05 7c c6  |s.............|.|
000000e0  44 ff 00 66 31 c0 88 f0  40 66 89 44 04 31 d2 88  |[email protected]..|
000000f0  ca c1 e2 02 88 e8 88 f4  40 89 44 08 31 c0 88 d0  |[email protected]...|
00000100  c0 e8 02 66 89 04 66 a1  44 7c 66 31 d2 66 f7 34  |...f..f.D|f1.f.4|
00000110  88 54 0a 66 31 d2 66 f7  74 04 88 54 0b 89 44 0c  |.T.f1.f.t..T..D.|
00000120  3b 44 08 7d 3c 8a 54 0d  c0 e2 06 8a 4c 0a fe c1  |;D.}<.T.....L...|
00000130  08 d1 8a 6c 0c 5a 8a 74  0b bb 00 70 8e c3 31 db  |...l.Z.t...p..1.|
00000140  b8 01 02 cd 13 72 2a 8c  c3 8e 06 48 7c 60 1e b9  |.....r*....H|`..|
00000150  00 01 8e db 31 f6 31 ff  fc f3 a5 1f 61 ff 26 42  |....1.1.....a.&B|
00000160  7c be 85 7d e8 40 00 eb  0e be 8a 7d e8 38 00 eb  ||..}.@.....}.8..|
00000170  06 be 94 7d e8 30 00 be  99 7d e8 2a 00 eb fe 47  |...}.0...}.*...G|
00000180  52 55 42 20 00 47 65 6f  6d 00 48 61 72 64 20 44  |RUB .Geom.Hard D|
00000190  69 73 6b 00 52 65 61 64  00 20 45 72 72 6f 72 00  |isk.Read. Error.|
000001a0  bb 01 00 b4 0e cd 10 ac  3c 00 75 f4 c3 00 00 00  |........<.u.....|
000001b0  00 00 00 00 00 00 00 00  b2 ec b2 ec 00 00 80 01  |................|
000001c0  01 00 07 fe 3f 34 3f 00  00 00 b6 fd 0c 00 00 00  |....?4?.........|
000001d0  01 35 0f ef ff ff f5 fd  0c 00 cc 39 ec 0d 00 00  |.5.........9....|
000001e0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
000001f0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 55 aa  |..............U.|

The solution is based on the fact that /dev/sda is a special device file which may be read.