Using existing home directory from a bootable external drive

Since the home directory on the external disk has it's own partition, that partition may be mounted at the /home/usernme on the SSD's system. Get the UUID of the filesystem on the home partition:

sudo blkid

and use that in a line appended to /etc/fstab

UUID=<whatever blkid output for it> /home/<username>   ext4    defaults        0       0

Replace ext4 in the above with whatever was used (ext3?, ext2?) for the actual filesystem.

On a 10 year old traveling disk, backups should be frequent. Monitor the smartctl -a output for the disk to see if problems are occurring (like the G-sense error rate). An SSD in the enclosure would avoid the mechanical problems, but I've had problems develop after 9 mo. on an encrypted section of an SSD (and you do encrypt the traveling disk, don't you?). Anyway, SSD over USB cannot do TRIM, so you'd need an esata port also and on your home system.