Well, I've answered my own question. I found this at the Adobe Lightroom support website.
Q) Can I store my library on a network drive?
A) No, this is not supported. There are too many variables in a network configuration to guarantee that the library database will not become corrupted. See the next question for how to use your library with multiple computers.
Here's some additional input about this problem.
Not only are there many variables that can affect reliability in a networked disk setup, but there are also serious performance issues in CIFS (the network file system used by Windows and therefore most home/small office NAS devices).
Since you don't mention which ReadyNAS product you're using, it's a bit hard to say exactly what your performance will be like, compared to a FireWire 400/800 or USB 2 drive, but you should expect lower performance regardless of how good your network is.
According to the ReadyNAS Comparison Chart
, the best theoretical performance you can get is 32 MB/s read and 24 MB/s write (if jumbo frames are enabled throughout a Gigabit Ethernet network).
The best theoretical performance for internal SATA drives is about twice as good, and with significantly better latency figures. (Latency is, in brief, the time between the request for data to be read or written, and the point in time where the data has been fetched or written.)
FireWire comes reasonably close to internal drives. USB 2.0 may
reach comparable performance to FireWire, but only if there are no other active USB devices on the same bus (sometimes the same controller).
The advice about looking at iSCSI is not unreasonable in itself, but keep in mind that iSCSI works better with dedicated network interfaces and cabling than when connected to a hub or a home office switch.
External SATA (eSATA) is becoming a competitive solution for home computers, but like FireWire and USB drives, the usable cable length is limited (I don't recall the exact numbers for either interface, sorry).
And if you're on a wireless network, you're in for more potential performance problems with network disks, depending on atmospheric conditions (yes, even inside your house) and radio activity (other wireless devices in your house or neighbouring houses).
All in all, for active image editing, NAS devices designed for home use are probably not the best for such purposes.
Getting decent performance from NAS or SAN (don't you love these acronyms?) is possible, however, it just costs money.