Backing up using eSata disk

Hi all,

There was a recent question on our form on using eSata disk for backup, specifically concerning AHCI, eject, differences between AHCI and USB.

Because it’s quite a hot topic, I’ll repost the question and our reply here, so subscribers to this blog can stay up to date 🙂

My big issue is with eSATA and AHCI. Under Linux, if you get the AHCI driver installed properly, eSATA drives act like your typical USB plug ‘n play device. “mount” and ‘umount” work exactly the same way. But if you do not have the AHCI drive properly installed (you are still using the ATA_PIIX driver), the mounting of a removed eSATA drive, or even seeking a removed eSATA drive’s status, will cause your computer to come down around your ears.

Now under Windows, you have to unmount an eSATA drive by uninstalling it in the device manager. The little green eject device arrow is not eSATA/AHCI aware. I have no idea what will happen under Windows if you do not have the proper ICH* driver loaded.

AHCI is not USB.

My concerns:
1) are you AHCI aware?
2) Do you specifically address the differences between AHCI and USB?
3) What happens if you seek a removed eSATA drive’s status?

Our answer to this:

I have to say that our own testing confirms exactly your observations about AHCI and eSATA. We’ve done specific testing on a variety of machines and motherboards, and found that when AHCI works, it works, but it seems to be the exception rather than the rule.

BackupAssist is actually able to function with eSATA regardless of whether you have AHCI enabled or not. It’s designed to simply avoid many of the problems that you would experience, because it will:

1. Before the backup, do a full hardware rescan, so that:
   1. If AHCI is enabled and a drive has been newly physically connected, the rescan has no effect because the drive is already present
   2. If AHCI is enabled and a drive has been soft-ejected (safely removed) then the rescan will inject it back into the system
   3. If AHCI is not enabled, any newly connected drives will be injected back into the system
2. After the backup, it will do a safely remove hardware:
3. Regardless of AHCI, the drive letter will disappear, and the device can be unplugged safely

Note that this method covers all cases that we know of, and appears to work flawlessly on all the hardware in our test lab (Server 2003 and 2008).

So to answer your questions

1. Yes, we’re AHCI aware, but it actually makes no difference for the reasons outlined above
2. Yes, we handle AHCI and USB, and we don’t even need AHCI to be activated in order for it to work. In the UI, if you click the “Detect” tab, it will do the hardware rescan to detect newly inserted drives.
3. This will never happen with BackupAssist because we eject the device after the backup.

Following from your Q3, if you DON’T use BackupAssist, and you unplug the device and AHCI isn’t installed, Windows sees a “ghost” copy of the drive and tries to cache changes, eventually getting into a big mess and rebooting or blue screening.

So the rule is: use BackupAssist’s features and there shouldn’t be any problems!