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<blockquote data-quote="MacInWin" data-source="post: 1836202" data-attributes="member: 396914"><p>Yes, that is what I said. Well, yes and no. The Mac knows that the boot drive is now on the external bus, not the internal bus because that is where the drive is physically attached and where the system has to look for the boot system. At the hardware level, the system knows the difference. But as long as the external drive responds in sufficient time the functions all work, albeit the slower external interface will make the machine seem sluggish in comparison. If the external drive doesn't respond in time the system will kernel panic with the hardware "failure" of the drive being reported. It will do the same kernel panic if the boot drive is the internal drive as well, but the higher speeds of the internal bus give the timing logic more flexibility in drive performance. So a drive that is having issues but is just barely acceptable internally as boot may well fail with a panic if booted in an external enclosure because of the slower interface speeds being added to the marginal response of the drive itself.</p><p>As I said, YMMV. I have actually seen drives that worked internally but would not boot in an external enclosure. I had to put that particular drive back inside and copy from there to the new drive as an external to get the data off of it because it would not boot in the exact same external enclosure to allow it to be cloned to the bare internal. PITA to do the swap twice (actually three times). So bus vs. interface speed can make a difference. With the advent of TB3 and USBc, the differences between internal and external interface speeds is shrinking. But the OP is talking about a very old Mac Pro that probably has original USB and Firewire 400 ports for external interfaces, (I don't know, just speculating) so booting from an external, compared to an internal, is going to be sluggish.</p></blockquote><p></p>
[QUOTE="MacInWin, post: 1836202, member: 396914"] Yes, that is what I said. Well, yes and no. The Mac knows that the boot drive is now on the external bus, not the internal bus because that is where the drive is physically attached and where the system has to look for the boot system. At the hardware level, the system knows the difference. But as long as the external drive responds in sufficient time the functions all work, albeit the slower external interface will make the machine seem sluggish in comparison. If the external drive doesn't respond in time the system will kernel panic with the hardware "failure" of the drive being reported. It will do the same kernel panic if the boot drive is the internal drive as well, but the higher speeds of the internal bus give the timing logic more flexibility in drive performance. So a drive that is having issues but is just barely acceptable internally as boot may well fail with a panic if booted in an external enclosure because of the slower interface speeds being added to the marginal response of the drive itself. As I said, YMMV. I have actually seen drives that worked internally but would not boot in an external enclosure. I had to put that particular drive back inside and copy from there to the new drive as an external to get the data off of it because it would not boot in the exact same external enclosure to allow it to be cloned to the bare internal. PITA to do the swap twice (actually three times). So bus vs. interface speed can make a difference. With the advent of TB3 and USBc, the differences between internal and external interface speeds is shrinking. But the OP is talking about a very old Mac Pro that probably has original USB and Firewire 400 ports for external interfaces, (I don't know, just speculating) so booting from an external, compared to an internal, is going to be sluggish. [/QUOTE]
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