How to change boot priority?
On some systems you have to press F2, Del or Esc to access Bios settings. Less frequently it is CTRL+Alt+ESC.
Once in Bios settings, on some systems, while there is a boot order giving you "removable drives" and "primary hard disk" as options, you may have to set the boot order again in an item labelled "HDD's" where you find again the USB drive.
The boot priority settings are often found under the Boot tab of the computer's BIOS/UEFI utility. The BIOS/UEFI utility can be accessed by pressing the key which is shown on the manufacturer's splash screen when booting. This is the F12 key on many computers. The keys to press, such as F12, F2, Del or Esc, differ on computers from different manufacturers. When grub bootloader is installed it does not change the key which is used to access the BIOS/UEFI utility.
When the power button is pressed to start the computer, it will try to boot first from whatever option is listed first in the boot priority section of the computer's BIOS/UEFI utility. If a USB flash drive is listed first in the boot priority list, it will try to boot from the USB flash drive first. If the internal hard drive where Ubuntu is installed is listed first in the boot priority list, the computer will boot to Ubuntu.
Boot menu and BIOS access keyssource
Manufacturer Models Boot Menu Key BIOS Key Acer Aspire One zg5, zg8, Aspire Timeline F12 F2 Acer Aspire v3, v5, v7 F12* F2 Acer Esc, F2, F12 Del, F2 Manufacturer Models Boot Menu Key BIOS Key Apple Post-2006 models Option Manufacturer Models Boot Menu Key BIOS Key Asus Desktops F8 F9 Asus VivoBook f200ca, f202e, q200e, s200e, s400ca, s500ca, u38n, v500ca, v550ca, v551, x200ca, x202e, x550ca, z202e Esc Delete Asus N550JV, N750JV, N550LF, Rog g750jh, Rog g750jw, Rog g750jx Zenbook Infinity ux301, Infinity ux301la, Prime ux31a, Prime ux32vd, R509C, Taichi 21, Touch u500vz, Transformer Book TX300 Esc (Disable "Fast Boot" and "Secure Boot Control") F2 Asus k25f, k35e, k34u, k35u, k43u, k46cb, k52f, k53e, k55a, k60ij, k70ab, k72f, k73e, k73s, k84l, k93sm, k93sv, k95vb, k501, k601, R503C, x32a, x35u, x54c, x61g, x64c, x64v, x75a, x83v, x83vb, x90, x93sv, x95gl, x101ch, x102ba, x200ca, x202e, x301a, x401a, x401u, x501a, x502c, x750ja F8 Delete Asus Eee PC 1015, 1025c Esc F2 Manufacturer Models Boot Menu Key BIOS Key Compaq Presario Esc, F9 F10 Manufacturer Models Boot Menu Key BIOS Key Dell F12** F2 Manufacturer Models Boot Menu Key BIOS Key eMachines F12 Tab, Delete Gateway Manufacturer Models Boot Menu Key BIOS Key Fujitsu F12 F2 Manufacturer Models Boot Menu Key BIOS Key Gigabyte F12 Del Manufacturer Models Boot Menu Key BIOS Key HP Esc, F9 Esc, F10, F1 HP Pavilion Media Center a1477c Esc F10 HP Pavilion 23 All In One Esc (Select boot media from the menu) F10 HP Pavilion Elite e9000, e9120y, e9150t, e9220y, e9280t Esc, F9 F10 HP Pavilion g4, g6 and g7, Probook 4520s, 4525s, 4540s, 4545s, 5220m, 5310m, 5330m, 5660b, 5670b Esc F10 HP Pavilion HPE PC, h8-1287c, Pavilion PC, p6 2317c, Pavilion PC, p7 1297cb, TouchSmart 520 PC, ENVY x2, m4, m4-1015dx, m4-1115dx, sleekbook m6, m6-1105dx, m6-1205dx, m6-k015dx, m6-k025dx, touchsmart m7, Envy, dv6 and dv7 PC, dv9700, Spectre 14, Spectre 13, 2000 - 2a20nr, 2a53ca, 2b16nr, 2b89wm, 2c29wm, 2d29wm Esc (Then F9 for "Boot Menu") Esc HP 2000 Esc (Then F9 for "Boot Menu". Select "Patriot Memory" on the Boot Option Menu) Esc HP Pavilion a410n Esc F1 Manufacturer Models Boot Menu Key BIOS Key Intel F10 Manufacturer Models Boot Menu Key BIOS Key Lenovo IdeaPad s300, u110, u310 Touch, u410, u510, y500, y510, yoga 11, yoga 13, z500 Novo Button (in the left corner of the laptop), F12 Novo Button Lenovo IdeaPad P500 F12 or Fn + F11 F2 Lenovo IdeaPad S10-3, g460, g470, g475, g480, g485 F12 F2 Manufacturer Models Boot Menu Key BIOS Key Microsoft Surface Pro 1-3 Volume-Down Button Microsoft Surface Pro 4 & Book Volume-Up Button Manufacturer Models Boot Menu Key BIOS Key NEC F5 F2 Manufacturer Models Boot Menu Key BIOS Key Packard Bell F8 F1, Delete Manufacturer Models Boot Menu Key BIOS Key Samsung F12, Esc F2, F10 Samsung NC10, np300e5c, np300e5e, np350v5c, np355v5c, np365e5c, np550p5c Esc F2 Samsung Series 5 Ultra, Series 7 Chronos, Series 9 Ultrabook Esc (Must disable "Fast Boot" in BIOS) F2 Samsung Ativ Book 2, 8, 9 F2 (Must disable "Fast Boot" in BIOS) F10 Manufacturer Models Boot Menu Key BIOS Key Sharp F2 Manufacturer Models Boot Menu Key BIOS Key Sony VAIO Duo, Pro, Flip, Tap, Fit Assist Button (Use the Assist Button when the computer is off, not when it is booting) Assist Button (Use the Assist Button when the computer is off, not when it is booting) Sony VAIO, PCG, VGN F11 F1, F2, F3 Sony VGN Esc, F10 F2 Manufacturer Models Boot Menu Key BIOS Key Toshiba Kira, Kirabook 13, Ultrabook, Qosmio g30, g35, g40, g50, Qosmio x70, x75, x500, x505, x870, x875, x880 F12 F2 Toshiba Protege, Satellite, Tecra F12 F1, Esc Toshiba Equium F12 F12 Manufacturer Models Boot Menu Key BIOS Key ASUS F8 DEL Gigabyte F12 DEL MSI F11 DEL Intel F10 F2 Asrock F11 F2, DEL EVGA F7 DEL
*F12 Boot Menu must be enabled in BIOS.
**Select USB Flash Drive.
If you can't get into the BIOS setup utility at all because it's password protected and you forgot the password you can remove the BIOS password by removing the CMOS battery for 5 minutes and then putting it back in. This will also reset all BIOS/UEFI settings back to their defaults.