Kernel not recognizing your hardware? Problems with power management or PCMCIA? What hardware is compatible with Gentoo? See here. (Only for kernels supported by Gentoo.)
Hum i pulled some Samsung SP2504C out of another box and with those and the Adaptec AAR-1210SA hot-plugging works.
However with the SCSI-drives i have to use:
echo "scsi remove-single-device HOST CHAN DEV LUN" > /proc/scsi/scsi
echo "scsi add-single-device HOST CHAN DEV LUN" > /proc/scsi/scsi