okay.... I have mint17.3 on the computer that it doesn't like to boot from. I have mint 18 on my laptop and I just plugged the external and got aninfo window that says:
Error mounting /dev/sdb1 at /media/michael/TravelStuff: Command-line `mount -t "ntfs" -o "uhelper=udisks2,nodev,nosuid,uid=1000,gid=1000" "/dev/sdb1" "/media/michael/TravelStuff"' exited with non-zero exit status 13: Incomplete multi-sector transfer: magic: 0x454c4946 size: 1024 usa_ofs: 48 usa_count: 0 data: 18 usn: 17: Input/output error
Record 0 has no FILE magic (0x44414142)
Failed to load $MFT: Input/output error
Failed to mount '/dev/sdb1': Input/output error
NTFS is either inconsistent, or there is a hardware fault, or it's a
SoftRAID/FakeRAID hardware. In the first case run chkdsk /f on Windows
then reboot into Windows twice. The usage of the /f parameter is very
important! If the device is a SoftRAID/FakeRAID then first activate
it and mount a different device under the /dev/mapper/ directory, (e.g.
/dev/mapper/nvidia_eahaabcc1). Please see the 'dmraid' documentation
for more details.
(fyi.... the external unmounted itself as I was copying files to it)