Drives with TLER are specifically designed for redundant RAID arrays. TLER is a drive function, where if the drive has difficulty accessing a portion of data, it will give up quickly and report an unreadable condition to the host controller. This timing is usually around 6-7 seconds maximum. The reasoning for this is that in a redundant array, all the data can be accessed or reconstructed from parity using other drives. The host controller then uses the remap function on the hard drive to mark those sectors bad, and writes the reconstructed data to spare sectors. This whole process happens seamlessly and without interruption or degradation of the RAID array. If it happens so many times that it runs out of spare sectors, the drive will be dropped from the array and marked as bad.
If you use a drive configured with TLER in a non-redundant configuration, such as a single drive or in a RAID0, the drive still acts the same - it quickly gives up reading the data. This will usually cause CRC or other errors to display in the operating system, and the data will not be able to be read. Naturally, this results in a loss of that data.
In contrast, a drive designed for use in desktop systems as single drives don’t have this timeout. Without TLER, the drive will literally try forever to get that data (and more often than not will eventually succeed). The operating system will appear to be extremely slow or frozen at this time, as it is waiting for the drive to become responsive again. If a drive without TLER is used in a redundant RAID, it will essentially cause the RAID to be degraded immediately upon hitting a single unreadable sector since the drive will appear to have become unresponsive to the host controller.
The short answer of it is…
To avoid unnecessary down time and headaches, don’t use a drive without TLER in a redundant RAID array.
It’s OK to use a drive with TLER in a non-redundant configuration, just be aware it will be more difficult to recover data from sectors that develop issues.