Windows 7 - Bringing a HDD online in Disk Mgmt does what to the MBR of that drive?

I needed to copy files between two PCs and didn't have a USB big enough so I took the HDD out of my Dell Vista laptop and used it as an external drive to transfer the files I needed moving. When I connected it up to the destination PC running windows 7, the drive couldn't be seen in explorer. I had to bring the drive online in Disk Management in order to be able to explore the drive and take the files I needed off of it. All was OK until I put the drive back in the Vista laptop and then found that the drive wouldn't boot. All I get is a blinking cursor after the initial Dell logo screen. like as if the BIOS can't load the drive or something.

Can someone please tell me what bringing a drive online in Disk Management does to the MBR of that drive?

I wouldn't think that simply attaching a drive to another PC and looking at it in disk management would do anything to the drive.

Did you do anything other than assign a drive letter to it?

What files did you take off of it? Did you move them off or copy them off?

When I hooked up the drive there were no letters assigned to the 4 partitions on it. When brought it online they were all assigned letters and then I could access it. I copied one 4.5GB file off it. I wasn't just a case of right clicking on a partition in Disk Management and assigning a letter to it. I had to bring the drive online by right-clicking on the drive itself and click 'Online'. Somebody was telling me that the drive might have been classed then as external storage by the windows 7 OS and did something to the MBR of the drive.

This was an internal drive on the original laptop.

You say you took it out and "used it as an external drive".

Do you mean you put it into an enclosure and used an external port? I don't use external drives, so if you did that, I don't know what might have occurred.

I'm also not familiar with "bringing a drive online".

Or did you connect it directly to an internal port on the second PC?

Apologies for the lengthy post...

I took the HDD out of a laptop and connected it up to another laptop via USB. The OS installed the necessary device drivers on connection but when I went to read the files on the disk I saw that the drive wasn't showing in My Computer.
I opened Disk Management and could see the drive listed but there was an exclamation mark displayed on the Disk name. When I clicked on it it opened a help article about having to import foreign drives into windows and bringing them online. I closed down the article and went back to Disk Management and when I right-clicked on the Disk Name, I had the option to bring the disk online. I did this and then all the partitions on the disk were assigned letters in Disk Management and the partition information was displayed. The disk could now be read in explorer so I copied my file off of it and then shutdown the laptop.
I disconnected the drive and put the drive back in my Dell Vista laptop, but The drive wouldn't boot. The initial Dell screen loads and then just a black screen with a blinking cursor. I have a feeling that bringing the drive online has done something to the MBR of the drive. Does anyone know if this is the case?

Does this help explain the term "online"?

To reactivate a dynamic disk by using the Windows interface

1. In Disk Management, right-click the disk marked Missing or Offline.
2. Click Reactivate Disk. The disk should be marked Online after the disk is reactivated.

Source: Windows Help