It is interesting to see that on the mainbaord of a 3/60 there is an empty place for a 3.5" hard disk. There are even holes for the screws, two rows of solder pads for a 50 pin SCSI connector and a solder pad with four holes for a power connector (+5, GND, GND, +12). I placed a Seagate ST31200N hard disk (1 GB) therein and made all necessary connections. The ST31200N has a height of only 1 cm and doesn't become very hot which makes it perfecly suitable for this purpose.
On the left you can see the two rows of the SCSI connector. Between the connector and the NEC 5380 SCSI controller chip there are three termination resisitors. If you want to use an internal disk and external devices, these have to be removed. For greatest flexibility, you should set them on sockets.
Hint: It is rather difficult to remove the terminator resistors without damaging them. Get replacement beforehand so to have no need to be careful but tear them out pin by pin.
The power connector does not carry +12V, but the ethernet fuse is right nearby. The fuse is 2A which is more than enough for a transceiver (200mA) plus a hard disk. Simply solder a wire from the fuse to the +12V pin of the power connector.
Another hint: The termpower lines of the 3/60 are not connected to +5V! Mostly they are not connected at all but on some 3/60 they are connected to ground! You will know, when some smoke arises. It is a good idea to jumper the internal disk to not provide termpower to the SCSI-bus but only to its internal resistors.