For various technical reasons it is not generally possible to adjust the standby timer of an IDE disk connected through a USB adapter (including turnkey external USB drives). See the sdparm page and the section below on sdparm for the technical reasons.
So one method available is to temporarily connect the drive via its IDE connector directly to a big Linux box and run hdparm to set the timer (the -S option is what you’re looking for, PLEASE read the hdparm man page before even thinking about running it).
The second method is to use scsi-idle. However, this requires a kernel patch as the corresponding kernel changes are not yet included in the nslu2-linux binaries. See the scsi-idle section below.