The moment you mount the new parition on top off ~/Documents
you cant access the Files anymore, but there are some Options:
Mount the HD somewhere else first and move the files. (code is just an example, it is not supposed to be executable as is)
mount /dev/disks/by-lable/foo /media/temporary
mv /home/hildogjr/Documents/* /media/temporary
umount /media/temporary
mount /dev/disks/by-lable/foo /home/hildogjr/Documents
Use a bindmount, to still be able to access the files after mounting:
mount --bind /home/hildogjr/Documents /media/Documents_on_ssd
mount /dev/disks/by-lable/foo /home/hildogjr/Documents
mv /media/Douments_on_ssd/* /home/hildogjr/Documents
umount /media/Documents_on_ssd
Use a unionfs, and move the files over Time. man unionfs
:
It first tries to access the file on the top branch and if the file
does not exist there, it continues on lower level branches. If the
user tries to modify a file on a lower level read-only branch the file
is copied to to a higher level read-write branch if the copy-on-write
(cow) mode was enabled.