Sophos and external hard disk drives

I see the previous message from Andrew, but I have my doubts:

I am on a MacBook Pro, aiming to use Sophos to scan the HD(s) of my iMac (which has become so slow as to be almost useless). It has one internal and 2 external HDs.

I started up the iMac in Target mode and Firewire cabled it to the Macbook, and an external HD icon with the Target mode symbol appears on

my Macbook desktop, with the same label as when the iMac runs normally.

  According to Andrew's message, then, "Scan Local Drives" means Sophos will scan the HD of my iMac. Right? 

I started the scan yesterday evening and left it running overnight. No threats appear to have been found (nothing in the Quarantine folder). Neither computer went to sleep.

But the 

total number of items scanned was about the same as when I had used "Scan Local Drives" on my Macbook alone. So I am suspicious of the claim that Sophos should have scanned the HD of my Targetmode/Firewire connected iMac as well as the HD of my Powerbook.

Also, this morning, Sophos displayed a message "This HD was last scanned on 19 Feb 2013"  !! (today is 18 July 2013)

***Perhaps what I was hoping to do is not possible with Sophos: scan the internal and external HDs of my iMac, using Sophos running on

my Powerbook. *** If so, how should I do a Sophos scan of these HDs? 

 I had earlier done a scan of my iMac with the Sophos installed on the iMac: no threats detected. Since the iMac

continues to run extremely slowly -- suggesting malware -- I wanted to use an external Sophos in case a virus had modified the local Sophos. Only then will I assume there is a hardware problem.. 

Any suggestions welcome!!

I see the previous message from Andrew, but I have my doubts:

I am on a MacBook Pro, aiming to use Sophos to scan the HD(s) of my iMac (which has become so slow as to be almost useless). It has one internal and 2 external HDs.

I started up the iMac in Target mode and Firewire cabled it to the Macbook, and an external HD icon with the Target mode symbol appears on

my Macbook desktop, with the same label as when the iMac runs normally.

  According to Andrew's message, then, "Scan Local Drives" means Sophos will scan the HD of my iMac. Right? 

I started the scan yesterday evening and left it running overnight. No threats appear to have been found (nothing in the Quarantine folder). Neither computer went to sleep.

But the 

total number of items scanned was about the same as when I had used "Scan Local Drives" on my Macbook alone. So I am suspicious of the claim that Sophos should have scanned the HD of my Targetmode/Firewire connected iMac as well as the HD of my Powerbook.

Also, this morning, Sophos displayed a message "This HD was last scanned on 19 Feb 2013"  !! (today is 18 July 2013)

***Perhaps what I was hoping to do is not possible with Sophos: scan the internal and external HDs of my iMac, using Sophos running on

my Powerbook. *** If so, how should I do a Sophos scan of these HDs? 

 I had earlier done a scan of my iMac with the Sophos installed on the iMac: no threats detected. Since the iMac

continues to run extremely slowly -- suggesting malware -- I wanted to use an external Sophos in case a virus had modified the local Sophos. Only then will I assume there is a hardware problem.. 

Any suggestions welcome!!