Sometimes, there are problems with iSCSI LUNs hosted on the NAS. iSCSI is a protocol of presenting some storage over the network (over the Internet for i) as a hard drive (a SCSI hard drive). i + SCSI = iSCSI. With NASes, some part of the NAS disk space is configured to be accessible over iSCSI. Each distinct piece of disk space presented over iSCSI is called LUN. The PCs connected to the NAS sees an iSCSI LUN same as its own physical drive. This is all you need to know about iSCSI, unless you want to dig deeper. If you want more, start with Wikipedia article.
There are two types of problems with iSCSI.
First type is illustrated on the QNAP forum – iSCSI LUN turned to RAW,
[I] have had iSCSI on my TS-212… Is see the iSCSI drive, but when I try to access the drive Windows says that drive needs to be formated before it will be used. When I checked the file system it says that disk is RAW instead of NFTS…
This case does not look like it requires any recovery on the NAS side. The faulty “disk” should be treated in the same was as one would treat a local drive. A plethora of software is available for you to choose from to work with RAW filesystem in Windows.
Second type of the problem is when the “disk”, that is, iSCSI LUN, is no longer visible, or cannot be “attached” to the PC. Depending on the exact setup of the NAS, recovery may be quite a complex process, beyond the scope of Home NAS Recovery. If you have this type of problem, you probably should be looking for a data recovery service.