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There are really two answers to your question, one hardware and one software.
Hardware: PCMCIA and USB connectors are designed to make power connections on insertion before signals and break power after signals on removal. This provides a relatively safe "hot plug". The other bus connectors (ISA, PCI, IDE/ATA) are not designed with this feature.
Software: PCMCIA and USB drivers are *designed* for recognition of hardware at any time. The other buses depend on a one-time scan for hardware during the boot procedure that identifies them and allocates resources accordingly.
It is interesting to note that SCSI is sort of in between the two. It is not technically a "hot plug" interface, since that would interfere with bus termination. But it is possible to have a SCSI device (such as a scanner) turned OFF during the boot-up initialization process, which you then turn ON and use at a later time. Windows suppports this, although you have to force a "Refresh" in the system device manager to have the scanner properly recognized.