You might want to run a disk check on those disks before using them again on your PS2.
If their filesystems were damaged, the chances of a crash on your PS2 would be probably quite high.
If you are using a USB HDD, try to give it some more power before connecting it to your PS2 (Either by using a Y-cable or by plugging in an external AC adaptor).
Unmodified SCPH-77006 with SM 3.6
SCPH-39006 with M-chip modchip, SCPH-10281 NA and refurb Seagate 80GB HDD
SCPH-10000 v1.00 with SCPH-10190 PCMCIA NA and SCPH-20400 HDD unit